To see the doodle in action just go to Google or check out this Youtube video: http://youtube.com/watch?v=4qtFeF6o82o. So there are 6 muppets for each letter in Google's logo. You can select one and it starts following your cursor moves and reacts on click.

There've been lots of videos appearing on Youtube claiming they reveal secrets and hidden animations of this doodle, so just let me summarise them and add some of my own thoughts.

In Google's homepage source code each muppet-letter has,well, a code, i.e. G is 1g, O is 2o, next O is 3o, G is 4g, L is 5l and E is 6e. All animations, no matter whether it's just following your cursor, or eating each other, are put into a .PNG image, which is a so-called 'sprite'.

Move animations for each letter are:

G

O

O

G

L

E

But those are just moves, and they are boring. Let's talk about hidden animations. Lucky we, they're "hidden" with the same naming pattern, so it was not that hard to find them. Knowing G is 1g, O is 2o and so on and doing some clicking-around-watching-what's-going-on we can get the following:

G

O

O

G

L

E

Note: to make the crazy-eyed red puppet eat another you select the long/tall green one (the L letter) then move your mouse pointer to the LOWER RIGHT so he's looking down at the red monster. CLICK RAPIDLY while holding this position, and the red monster will eat him. (source)